Slows2k

This is a very bad idea. MTF isn't anywhere close to the correct weight oil. You'll find out quickly when the gears destroy themselves, usually punching big holes in the rear housing. The final drive gears in a FWD trans have very different lubrication needs vs a RWD final drive. Just ask those who have used VTM-4 fluid and turned the gears to shrapnel.

stitos2k

Eh... honda MTF is about the same as 10w30 motor oil in thickness... You want to put that in a diff the REQUIRES a 90w fluid?

Many a diff has gone BOOM from dingus dealerships doing similar things as that, and it usually only takes about 500 miles before it goes.

But hey, its your diff and $2000 repair bill.

SpitfireS

When oil viscosity is rated it is important to know what kind of oil it is.
Engine oil specs are determined by the SAE J300 list and gear oil specs are determined by the SAE J306 list.

Water, (H2O) at 20.2C has a vicosity of 1 cSt.
To give you an idea of how thick/thin it is.

Lets take the SAE 10W-30 engine oil:
At operating temp it has a viscosity in between 9.3 and 12.5 cSt.
(according to SAE J300 specifications)

When you look at SAE 90 gear oil for example:
At operating temp it has a viscosity in between 13.5 and 18.5 cSt.
(according to SAE J306 specifications)

There is not much difference between the two.
A 75W-85 gear oil has an operating temp viscosity in between 11.0 and 13.5 cSt.
That is in the range of the 10W-30 engine oil.
(and that is why you are allowed to use it in your transmission if you really have to)

So operating temp viscosity wise there is not much difference.
There is a huge and important difference in additives.
Those additives make the difference between engine oils and gear oils.
Honda MTF is a gear oil.
I do not know the GL rating of it, SAE spec, or any other.
And IMO the hypoid final drive gears will not last long with it => too much shearing going on, you need a proper GL-5 oil for that.

As mentioned before... test it and you will know

Ek9

I'm not suggesting you're wrong. I'm just thinking about the millions of "difs" in FWD cars that work properly under 10w30 or even ATF. It's not clear to me why a change in the cut of the gear requires 90 wt oil. If the answer it's there's alot more heat buildup in the conventional diff than on a transaxle or that the cut of the gears itself causes higher pressures that only a 90 wt can handle then that would make sene.

As for the 10w30 in the transmission, that was THE oil to use in all honda transmissions before MTF was created. I think it was the mid 90''s that MTF came out? I'm not certain. I know the '95 EG civic speced 10w30 as the transmission fluid of choice. Nissan FR transmissions from the same era used GL8 or something like that.

I'm not actually suggesting doing this. Just trying to satisfy my curiosity.